The present invention relates to polyurethane polyanhydride oligomers, a method for their preparation and their use in ambient temperature curable coating compositions to cure polyols.
In the area of automobile repair and refinishing, the capability of the coating to harden at ambient temperature is required when fully or partially repainting vehicles. Recognized products for this purpose include two package coating compositions based on, for example, hydroxyl functional materials such as acrylic polyols, and polyisocyanate curing agents. Polyisocyanate curing agents, however, have caused sensitization in a number of individuals, therefore, precautions must generally be taken. Also, particularly in the preparation of clear coating compositions, it is very important that the clear coating be transparent and non-yellowing.
Some particularly advantageous compositions from the perspective of low temperature cure, such as those described in U.S. Pat. No. 4,452,948 which are based on an anhydride hydroxyl curing mechanism, tend to yellow badly. This feature can be a significant detriment when color matching with an old finish is critical. The tendency to yellow is even more of a detriment when a clear topcoat composition is required for a color plus clear application method. This method involves coating a substrate with one or more applications of a pigmented basecoating composition to form a basecoat and then coating the basecoat with one or more coats of a transparent topcoating composition to form a transparent or clear topcoat. Should the clear topcoat yellow, the appearance of the vehicle is diminished.
There is a need therefore, for a non-isocyanate based clear coating composition, exhibits excellent physical properties, does not yellow and at least in some measure, reduces problems associated with sensitization.